Bacteriophage-like particles associated with the gene transfer agent of Methanococcus voltae PS

This study identified bacteriophage-like particles associated with the gene transfer agent (VTA) of the archaeon Methanococcus voltae strain PS. VTA is a filterable, DNase-resistant agent that transfers small DNA fragments (approximately 4 kb) between bacterial cells through a transduction-like mechanism, despite lacking apparent viral replication machinery. Using electron microscopy on two independently prepared and purified VTA samples, researchers observed virus-like particles with isometric heads approximately 40 nm in diameter and tails approximately 61 nm long. The particle dimensions were determined using bacteriophage T4 tail spacing as a reference. These particles co-sedimented with bacteriophage φX174 in sucrose density gradients, indicating similar sedimentation rates. The head size is consistent with packaging of the 4 kb DNA fragments characteristic of VTA. The findings suggest VTA represents a defective viral system where small chromosomal DNA fragments are encapsidated into bacteriophage-like structures capable of mediating gene transfer, a mechanism previously documented in only one other archaeon and a few other organisms.

Key findings

• Electron microscopy revealed bacteriophage-like particles with 40 nm diameter heads and 61 nm long tails associated with the VTA gene transfer agent of M. voltae
• VTA particles sedimented at rates similar to φX174 bacteriophage in sucrose gradients, indicating comparable particle sizes
• The particle head dimensions are consistent with packaging approximately 4 kb of double-stranded DNA fragments
• VTA represents a defective viral system capable of transducing small chromosomal DNA fragments despite lacking apparent complete viral genome replication machinery
• This type of gene transfer system with similarly small DNA fragments had been reported in only one other bacterium (Rhodobacter) and one eukaryotic virus prior to this study